An electrophotographic photosensitive member (hereinafter referred to also as “photosensitive member”) for use in an electrophotographic image-forming method generally has a photoconductive layer (photosensitive layer) and a surface layer formed on the photoconductive layer. In addition, when the photosensitive member is requested to form images at high speeds, an amorphous silicon (hereinafter referred to also as “a-Si”) and a hydrogenated amorphous silicon carbide (hereinafter referred to also as “a-SiC”) are often used in the photoconductive layer and the surface layer, respectively. The surface layer formed of the a-SiC is excellent in abrasion resistance, charge-retaining performance, and light permeability.
However, when the surface layer formed of the a-SiC is used under a high-humidity environment, image deletion (which may hereinafter be referred to as “high-humidity image deletion”) may occur. The high humidity image deletion refers to such an image defect that when image formation is repeatedly performed in a high-humidity environment and an image is output again after a certain time period from the completion of the repetition, characters blur or are not printed to bring about blank areas.
Moisture adsorbing to the surface of the photosensitive member is known to be one cause for the occurrence of the high humidity image deletion, so the following procedure has been conventionally adopted in order that the occurrence of the high humidity image deletion can be suppressed. The photosensitive member is heated with a photosensitive member heater at all times so that the moisture adsorbing to the surface of the photosensitive member can be reduced in amount or removed.
However, in the case where the photosensitive member heater is actuated at all times, considerable power is consumed as standby power even when an electrophotographic apparatus including the photosensitive member does not operate. As a result, an environmental load increases, and a running cost also increases.
Japanese Patent No. 3,124,841 discloses a technique involving making the atomic density of silicon atoms, carbon atoms, hydrogen atoms, or fluorine atoms in the surface layer formed of the a-SiC of a photosensitive member smaller than a predetermined value as a technique for suppressing the occurrence of the high humidity image deletion. The technique disclosed in Japanese Patent No. 3,124,841 is a technique for suppressing the occurrence of the high humidity image deletion, in which the surface layer of the photosensitive member is provided with such a coarse membrane structure that the surface of the photosensitive member is easily scraped off by a cleaning step so that a new surface adsorbing a small amount of moisture may be always formed on the surface of the photosensitive member.
However, the technique disclosed in Japanese Patent No. 3,124,841 involves the following problem. Because the surface layer of the photosensitive member is apt to be scraped off in association with its repeated use, the lifetime of the photosensitive member or of an electrophotographic apparatus including the photosensitive member is short (the durability of the photosensitive member or the apparatus is low).
In addition, a ghost is one of the problems concerning the electrophotographic image quality. The ghost is such a phenomenon that the history of an electrostatic latent image remains on a photosensitive member, so the pattern of an image previously output is output to overlap an image (toner image) transferred onto a transfer material.
A pre-exposing step of irradiating the surface of a photosensitive member with pre-exposure light to remove charge on the surface of the photosensitive member is often included in a conventional electrophotographic image-forming method after a transferring step and before a charging step with a view to suppressing the occurrence of the ghost. The peak wavelength of the pre-exposure light and the peak wavelength of image exposure light for forming an electrostatic latent image on the surface of the photosensitive member are germane to the potential characteristic of the photosensitive member and the quality of an image formed with the photosensitive member. Accordingly, the peak wavelength of the pre-exposure light and the peak wavelength of the image exposure light are properly selected in accordance with an electrophotographic image-forming method to be applied. Japanese Patent Application Laid-Open No. S58-080656 discloses, as an example of such method, an electrophotographic image-forming method in which laser light having a wavelength of 700 nm to 900 nm is used as the image exposure light and light obtained by cutting off light having a wavelength of 600 nm or more with a filter is used as the pre-exposure light so that an increase in dark current (reduction in charging performance of the photosensitive member) can be suppressed. In addition, Japanese Patent Application Laid-Open No. H08-022229 discloses an electrophotographic image-forming method in which light having a wavelength of 670 nm or more is used as the image exposure light and light having a wavelength of 620 nm or more is used as the pre-exposure light so that a state in which the depth of penetration of the pre-exposure light becomes shallow can be suppressed and the ghost can be removed.